Introduces shared <Modal>, <ModalHeader>, <ModalBody>, <ModalFooter>,
and <ModalDestructiveHeader> primitives that wrap shadcn Dialog and
encapsulate the canonical modal chrome: cyan rail at the left edge,
mono uppercase kicker, italic serif title, footer hint, standardized
button order (secondary outline, primary cyan / destructive).
Migrates NewFolderDialog and DeleteFileConfirm onto the new primitives
as the first proof points. The destructive variant flips the rail and
kicker color to the destructive token without changing the chrome
recipe.
Anso